Mosquito season in Mountrail County, ND: when it starts and ends

Mosquito months in Mountrail County

In a normal year mosquito season in Mountrail County, ND starts around May 11 and ends around September 27, about 20 weeks.

1. Jan
2. Feb
3. Mar
4. Apr
5. May
6. Jun
7. Jul
8. Aug
9. Sep
10. Oct
11. Nov
12. Dec

What to do in Mountrail County, and when

• Start tipping out standing water weekly by April 27.
• Book any yard treatment for mid May, ahead of the July peak.
• Repellent can go away after September 27.
• 20 West Nile cases since 2016: you should wear EPA-registered repellent at dusk July, and check ND spray notices.

Mountrail County against the rest of ND

Mountrail County has the 48th longest season of 53 ND counties, at 139 days against 160 in Sheridan County and 135 in Rolette County. Its mean temperature is 39°F.

• Mercer County: May 11 to September 28 (140 days, 25 WNV)
• Williams County: May 11 to September 28 (140 days, 31 WNV)
• Burke County: May 10 to September 25 (138 days, 2 WNV)
• Cavalier County: May 11 to September 26 (138 days, 1 WNV)

West Nile in Mountrail County

From 1999 to 2025, CDC ArboNET counted 37 human West Nile cases in Mountrail County (9 neuroinvasive) across 10 years. The worst was 2018 with 7; the latest was 2025.

See how the dates are worked out, and reading the West Nile counts.